The present invention concerns a method of controlling a stepping motor having a coil and a rotor which comprises a permanent magnet magnetically coupled to the coil each time the rotor has to turn by one step and measuring the quantity of mechanical energy supplied by the motor during the drive pulse.
The invention also concerns a device for carrying out this method.
Most stepping motors being used in small devices such as timepieces are controlled by drive pulses of a fixed duration which is sufficiently long for the rotor of these motors to turn by one step in response to each such pulse, even if the resisting torque applied to the rotor is equal to the maximum torque the motor can produce.
Controlling stepping motors in this way is very simple and implementation of the method does not require complicated means.
But when a stepping motor is controlled in this way, it consumes a large quantity of electrical energy because during the vast majority of drive pulses the resisting torque applied to its rotor is much smaller than the maximum torque it can produce.
Furthermore, the operating reliability of a stepping motor controlled in this simple manner is not very high, because the drive pulses it receives may in some instances produce a rotation of the rotor by more than one step or even cause the rotor to move back to its initial position.
To increase the useful life of the batteries supplying the electrical energy required for operation of the aforementioned devices, numerous methods have been proposed for reducing the quantity of electrical energy consumed by these motors by indirectly measuring, during each drive pulse, the resisting torque applied to their rotor and interrupting the drive pulse as a function of this measurement.
Thus, for example, U.S. Pat. No. 4772840 describes such a method of controlling a stepping motor. According to this method, the mechanical energy supplied by this motor is measured during each drive pulse, as well as the time taken by this mechanical energy to reach a reference value, which time depends on the resisting torque applied to the motor's rotor during the drive pulse.
The optimum duration of the drive pulse is determined as a function of the measured time, and the drive pulse is interrupted at the end of this optimum duration.
This method theoretically enables the quantity of electrical energy consumed by a stepping motor to be reduced to the smallest possible value, whatever may be the resisting torque applied to the motor's rotor.
This method however has the drawback that the relationship between the time taken for the mechanical energy supplied by the motor during a drive pulse to reach the reference value and the optimal duration of this drive pulse depends on the electrical and magnetic characteristics of the motor.
This relationship must thus be determined experimentally for each type of motor, which is a long and expensive operation.
It is also known that a stepping motor's rotor is submitted to a positioning torque tending to hold it in or to return it to either of its rest or stable equilibrium positions, and that during each rotation it makes in response to a drive pulse, the rotor passes through an unstable equilibrium position situated substantially halfway between the stable equilibrium position it has just moved from and the one it should move to.
But, when a stepping motor is controlled according to the aforementioned method, at the end of a drive pulse its rotor has moved through only about a third of the angular distance separating its initial rest position, that it has just left, from its final rest position, i.e. the position it should reach. The rotor has thus not yet reached its unstable equilibrium position situated between these two rest positions, and the positioning torque applied thereto opposes its rotation and tends to return it to its initial position.
Normally, the kinetic energy of the rotor at the end of a drive pulse is sufficient to overcome this positioning torque and bring the rotor at least to its aforementioned unstable equilibrium position where the positioning torque cancels out, changes direction and causes the rotor to rotate to its final rest position.
But if the resisting torque applied to the rotor increases abruptly, due to a shock for example just after the end of the drive pulse, there is a high risk that the rotor's kinetic energy will be insufficient to bring it to its unstable equilibrium position, and that the rotor thus returns to its initial rest position.
Moreover, the characteristics of various components of the devices for controlling these motors in carrying out this method may also differ from one device to another, or may vary as a function of time and/or of various factors which influence these devices.
These variations may cause the rotor of a motor controlled according to the aforementioned method to turn incorrectly in response to a drive pulse, even if the resisting torque applied to the rotor during and/or after the drive pulse is less than the maximum torque the motor can produce.
It can be seen that the operating reliability of a motor controlled according to the aforementioned method is not as high as could be hoped for.
To improve this operating reliability, the control devices for implementing this method must be provided with a circuit able to detect the eventual non-rotation of the rotor which is connected therewith and, following such detection, to apply to the motor so-called catch-up pulses during which the electrical energy supplied thereto is at least equal to the energy it needs to bring its output torque to its maximum value.
These catch-up pulses however have the drawback that they may in certain cases cause the motor's rotor to rotate by more than one step, or even make the rotor go back to its starting position. In such cases, the motor's reliability is obviously not improved.
Furthermore, considerable progress has been made in the design and manufacture of stepping motors, their electronic control circuits and the sources, batteries or accumulators, that supply the requisite electrical energy to operate these motors and circuits. As a result, it is nowadays not so critical as in the past to reduce as far as possible the consumption of the stepping motors used in devices of very reduced dimensions such as electronic wrist watches.